(1) Field of the Invention
This invention relates to a photoelectric sensor which is provided with a light collecting optical system and a photoelectric element.
(2) Description of the Prior Art
According to a conventional photoelectric sensor, a convex lens is disposed in a circular opening which is formed on a front wall of a box-shaped casing, and a luminescent diode or a photoelectric element such as a photo transistor is positioned on a focal point of the convex lens.
FIG. 5 shows a section view of such a conventional photoelectric sensor, in which symbol A denotes a convex lens disposed on front wall of a casing B. Numeral 21 denotes a photoelectric element positioned on a focal point of the convex lens A. Numeral 22 denotes a wiring substrate, while numeral 23 denotes various electronic parts mounted on the wiring substrate 22. Numeral 24 denotes a motion indicator lamp. Symbol C denotes a cable connecting both the motion indicator lamp 24 and the wiring substrate 22. Further, numeral 3 denotes a rear panel of the casing B.
In recent years, the structure of the photoelectric sensor becomes more compact and more small-sized year by year to respond to the users' requirements. However, there exist two problems which hinder a production of a compact photoelectric sensor.
Firstly, an assembling efficiency of the photoelectric sensor becomes lower as the structure of the photoelectric sensor casing becomes smaller. More specifically, when installing various electronic components on an inner wall of the casing, it is required to carry out such an assembling by inserting a driver, pincers, a soldering iron and other tools into the interior of the casing. Accordingly, as the structure of the photoelectric sensor becomes smaller, the assembling work becomes more cumbersome.
Secondly, since the convex lens is employed to collect lights, there is demanded a considerably broader space, i.e. a conical space for collecting the lights toward the photoelectric element positioned on a focal point of the convex lens.
In other words, in order to enhance the performance of the photoelectric sensor, it is desirable to enlarge a caliber of the convex lens and increase the collecting amount of the lights. Thus, a focal distance of the convex lens becomes longer, whereby the effect of any outer disturbing light becomes less. Therefore, a convex lens having a larger caliber and a long focal distance, as well as a broader conical space is necessitated.
As discussed above, up to now a desire to make more compact the photoelectric sensor casing is inconsistent with the betterment of its optical performance.